Lead-acid batteries are generally classified into flooded type and valve-regulated type and, since they have a feature of being inexpensive and highly reliable, they are generally used respectively as power sources for automobile starting, power sources for electromotive vehicles such as golf carts and, further, as power sources for industrial equipments such as uninterruptible power supplies.
In automobiles, various countermeasures have been studied in recent years for the improvement of fuel cost for preventing atmospheric pollution and preventing global warming. For automobiles applied with the countermeasure for the improvement of the fuel cost, studies have been made on micro hybrid vehicles such as idling stop and start vehicles (hereinafter referred to as ISS vehicles) for decreasing the engine operation time and generation control vehicles that efficiently utilize engine rotation to power.
In ISS vehicles, the number of engine starting times is increased and large current discharge from lead-acid battery is repeated on every time. Further, in the ISS vehicles or the generation control vehicles, since the amount of electric power generated by an alternator is decreased and the lead-acid battery is charged intermittently, the battery is often charged insufficiently. Accordingly, in the lead-acid battery used for the applications described above, a performance capable of performing charging as much as possible in a short time, that is, improvement for charge acceptance is required.
While it is necessary for valve-regulated lead-acid batteries to be mounted in a trunk room or the like where temperature is not high, they have advantages such as free of maintenance, long life, and vibration resistance compared with flooded lead-acid battery, and they have been mounted frequently in ISS vehicles made in Europe.
The lead-acid batteries used in the manner described above are used in a partially charged state referred to as PSOC (Partial State Of Charge). When the valve-regulated lead-acid battery is used under PSOC, the life tends to be shortened compared with the case of use in a completely charged state. It is considered that the life is shortened when used under PSOC because when charge and discharge are repeated in the insufficiently charged state, lead sulfate formed on the negative electrode upon discharge is grown and lead sulfate is less reduced to lead as the charging product. Accordingly, in the valve-regulated lead-acid battery used under PSOC, it is necessary, also for extending the life, to improve the charge acceptance (enabling charge as much as possible in a short time), thereby preventing repetitive charge and discharge in an excessively insufficient charged state to suppress lead sulfate from growing by repetitive charge and discharge.
As described above, in recent lead-acid batteries for vehicles, it is an extremely important subject to enable high rate discharging to the load by a short time charge, as well as improve the charge acceptance for improving the life performance of a battery when used under PSOC.
In the valve-regulated lead-acid battery, since the charge acceptance of a negative electrode active material is poor while charge acceptance of the positive electrode active material is high by nature, it is necessary to improve the charge acceptance of the negative electrode active material for improving the charge acceptance of the valve-regulated lead-acid battery. Therefore, efforts have been made exclusively so far to improve the charge acceptance of the negative electrode active material. Relevant related arts are shown below.
Patent Literature 1 and Patent Literature 2 propose to improve the charge acceptance by increasing the amount of a carbonaceous electroconductive material to be added to the negative electrode active material, thereby improving the life of lead-acid batteries used under PSOC. The proposals are directed to the valve-regulated lead-acid batteries as a target.
On the other hand, in the valve-regulated lead-acid batteries, organic compounds having an effect of suppressing growing of the negative electrode active material have been added to the negative electrode active material for keeping a high reactivity state in the charge and discharge reaction thereby suppressing growing of the negative electrode active material during charge and discharge and suppressing the decrease of the surface area of the negative electrode. Heretofore, as the organic compound for suppressing growing of the negative electrode active material, lignin which is a main ingredient of wood materials has been used. However, since the lignin has various structures in which multiple unitary structures are bonded in a complicate manner and usually has portions tending to undergo oxidation or reduction such as carbonyl groups, the portions are decomposed by oxidation or reduction during charge and discharge of the valve-regulated lead-acid battery. Therefore, even when the lignin is added to the negative electrode active material, the effect of suppressing the lowering of the performance by repetitive charge and discharge could not be maintained for a long time. Further, since the lignin is adsorbed to lead ions dissolving from lead sulfate to lower the reactivity of the lead ions upon charging, this results in a side effect of inhibiting the charge reaction of the negative electrode active material to suppress the improvement of the charge acceptance. Accordingly, the lignin added to the negative electrode active material involves a problem of hindering the improvement of the charge acceptability although it enhances the discharge characteristic.
With a view point described above, it has been proposed to add, instead of the lignin, sodium lignin sulfonate in which a sulfonic group is introduced to the a-position on the side chain of a phenyl propane structure which is a basic structure of the lignin, a formaldehyde condensate of bisphenols and aminobenzene sulfonic acid, etc. to the negative electrode active material.
For example, Patent Literatures 3 and Patent Literatures 4 disclose to add a folmaldehyde condensate of bisphenols aminobenzene sulfonic acid and a carbonaceous electroconductive material to the negative electrode active material. Particularly, the Patent Literatures 4 discloses that the effect of suppressing lead sulfate from growing is maintained by selecting the formaldehyde condensate of bisphenols aminobenzene sulfonic acid as an organic compound of suppressing lead sulfate from growing accompanying charge and discharge and that a carbonaceous electroconductive material is added for improving the charge acceptance. Further, Patent Literature 5 discloses to improve the discharge characteristic under PSOC by adding an electroconductive carbon and an activated carbon to the negative electrode active material.
Further, Patent Literature 6 discloses to increase the capacity by increasing the specific surface area of a positive electrode active material from 4.5 m2/g in the existence case to 6 m2/g at the greatest. This intends to make the positive electrode active material finer thereby increasing the specific surface area by adding the lignin in an electrolyte during formation of the positive electrode plate. However, according to an experiment, the method of the Patent Literature 6 involves a problem that softening of the positive electrode active material tends to proceed and involves a subject in the cycle life. Further, what is disclosed by the Patent Literature 6 is an invention of increasing the capacity of a battery and no remarkable effect can be obtained for the improvement of the charge acceptability and the cycle characteristic under PSOC necessary for the valve-regulated lead-acid battery for ISS vehicles or generation control vehicles.